1. Field of the Invention
The present invention relates to an improvement in a rolling bearing such as a ball bearing, a tapered roller bearing, a cylindrical bearing, a self-aligning roller bearing, or a thrust bearing, and in particular, to a rolling bearing capable of performing satisfactorily during severe conditions in which the feeding of lubricating oil is frequently interrupted during rotation at high speeds by controlling the relative movement between a raceway ring and the rolling elements.
2. Description of the Prior Art
The longevity of a rolling bearing is closely related to the degree of formation of a lubricating oil film on the rolling contact surface between the raceway ring and rolling elements of the bearing.
Accordingly, heretofore, several proposals have been made as to the formation of the lubricating oil film on the rolling contact surface of the bearing.
For example, in Japanese Utility Model Publication No. 49-40208, at least one of an outer ring, an inner ring, and the rolling elements of a rolling bearing has a shallow spiral-shaped waviness formed therein by grinding, superfinishing, or polishing. The shallow spiral-shaped waviness, on the order of a micron, does not affect the load capacity of the bearing. As a result, when the bearing is rotated, the spiral-shaped waviness acts as a screw pump and the lubricating oil is spread uniformly over the whole contact surface thereby forming an oil film.
In Japanese Patent Laid-Open Publication No. 62-274115 and Japanese Utility Model Laid-Open Publication No. 61-23520, a cross or mesh pattern having a depth of 0.1 to 0.4 .mu.m is formed by superfinishing in either or both of a raceway surface of a raceway ring and rolling element surface, so that convex portions thus formed are disposed, and the concave portions are connected serially in the axial direction and in the circumferential direction, thereby to enhance the oil film holding function.
However, in the former case, it is intended that the lubricating oil be uniformly distributed over the whole contact surface between the bearing raceway and the rolling elements due to the action of the shallow, spiral-shaped waviness. It is an aim, in such a structure, to solely improve the lubricating property based on the premise that the bearing is to be used in a normal, continuously oil fed condition. In other words, it is not intended that the bearing be used in a severe condition in which the supply of lubricating oil is frequently interrupted. Furthermore, a problem is posed in that it is very difficult to form the spiral-shaped waviness at a cost which is economically reasonable.
On the other hand, in the latter case, it is intended to improve the retention of the lubricating oil film during transportation of the bearing or at the start of operation of the bearing. It is the aim to exclusively solve the problem of impressions resulting from vibration of the bearing during transportation. In other words, in the bearing which is assembled to equipment and before the bearing is used in its operating condition, a sufficient lubrication oil film is not yet formed on the contact surface between the raceway ring and the rolling elements. As a result, impressions are caused by repeated tapping at the contact portion between the raceway ring and the rolling elements due to vibrations during transportation, resulting in an increase in the vibrations and sound of the bearing during use. Thus, it is desirable to prevent such a phenomenon. Specifically, the convex and concave portions in the cross shape, produced by the superfinishing in the latter case, are required at the time of transportation or the start of operation of the bearing before the bearing is used in its rotating condition. Once the bearing is assembled to equipment and driven into rotation, the concave and convex portions are not necessary. Accordingly, the depth of the cross-shaped pattern is restricted to a depth of, for example, 0.1 to 0.4 .mu.m which is flattened with the lapse of time by contact surface pressure between the raceway ring and the rolling elements.
However, recently, in high technical fields, the working conditions for bearings are becoming very severe. There is a need for a bearing which is not damaged such as by seizure or the like even under severe conditions in which, for example, the supply of lubrication oil is interrupted repeatedly for certain short times, such as 30 seconds, for a bearing rotating at high speeds under oil lubrication.
The general behavior of a bearing in a condition in which the supply of lubrication oil is temporarily interrupted is considered as follows.
Generally, in a bearing rotating at high speeds, the outer ring side is burdened with greater load, due to the centrifugal force acting on the rolling elements, as compared with the inner ring side. In a usual high speed operating condition, in which the lubricating oil is continuously supplied normally, since the lubricating oil is sufficient it may be considered that a satisfactory oil film is formed between the raceway ring and the rolling elements. As a result, there is no large difference in friction coefficient between the outer ring side and the inner ring side, and the frictional force exerted to the outer ring is larger than that exerted to the inner ring side by the amount of the rolling element load which is applied more to the outer ring side than the inner ring side. This allows the rolling elements to rotate in a substantially pure rolling condition at the outer ring side.
Supposing that the supply of the lubricating oil is interrupted temporarily, in a usual bearing which is used with its inner ring in rotation, since the lubricating oil on the surface of the inner ring flows off due to centrifugal force, the oil film on the inner ring side disappears and thus, the degree of metal contact is increased. As a result, the friction coefficient of the inner ring side increases rapidly. On the other hand, since there is no such centrifugal force acting on the outer ring side, the lubricating oil remains. Accordingly, the metallic contact portion is small at the outer ring side, and rapid increase of the friction coefficient is not caused as compared with the inner ring side.
For this reason, after a certain time elapses during which the supply of lubricating oil is interrupted, the frictional force of the inner ring side exceeds that of the outer ring side. At this time, the rolling elements rotate substantially in a pure rolling condition at the inner ring side, and simultaneously, rotate in a condition with a slight slip at the outer ring side. When the rotating conditions change to those described above, due to the interruption of the lubricating oil, a large skew and a gyro-slip occur causing the movement of the rolling elements to become unstable. At the same time, since revolution and rotation about their own axes are caused, heat is generated at the contact portion.
Because of the heat generated at the contact portion, the temperature of the rolling elements which have a small thermal capacity rises rapidly, and the tack strength of the oil at the surfaces of the rolling elements is weakened. Consequently, the supply of lubricating oil from the outer ring side to the inner ring side which has been effected through the rotation of the rolling elements becomes scarce, and the oil film is broken between the rolling elements and the inner ring resulting in seizure.
Such a seizure due to the unstable movement of the rolling elements occurring because of the interruption of the lubricating oil during high speed rotation occurs easily in the case of a typical bearing when the roughness of the raceway surfaces of the inner and outer rings is 0.01 to 0.08 .mu.mRa. In contrast, when the roughness of the above-mentioned raceway surfaces is made to be equal to or greater than 0.08 .mu.mRa, it is somewhat advantageous since the friction coefficient can be increased to such an extent that the unstable movement of the rolling elements is restricted. However, in this case, since the minute concave and convex portions, which constitute the roughness on the raceway surfaces, are flattened during repetitive interruption of the lubricating oil, seizure will occur in a similar fashion to the case in which the roughness of the raceway surfaces is equal to or smaller than 0.08 .mu.mRa. On the other hand, if the roughness of the raceway surfaces is made to be extremely rough, seizure will easily occur due to the heat generated by friction.